Such electromagnetic axle counters are used to indicate track occupancy or nonoccupancy in railway systems. Recent designs are described, for example, in articles published in "Signal+Draht" 77 (1985), No. 4, pages 72 et seq., and in "Signal+Draht" 78 (1986), No. 12, pages 264 et seq.
In all known designs of electromagnetic axle counters, the tires and/or flanges of the vehicle wheels act on an electromagnetic field which is generated by a transmitting coil in a transmitting head of a rail-mounted sensor portion of the axle counter and received by a receiving coil in a receiving head of the sensor portion, which is located a few centimeters from the transmitting head. The sensor portion, consisting of transmitting head and receiving head, including the transmitter feeding the transmitting coil and the receiver picking up the signal induced in the receiving coil, is also referred to as "electromagnetic rail contact".
The signal change sensed by the receiving coil upon passage of a wheel depends on predetermined parameters, but also on quantities which vary with time and necessitate regular readjustments of the rail contact.
The latter applies particularly to rail wear, as a result of which the flanges of passing wheels move along a lower path and, thus, closer to the transmitting head and receiving head and influence the electromagnetic field more strongly, which leads to receiver output signals of changed shape and duration.
In conventional axle counters, the changes caused by rail wear are compensated for by repositioning the transmitting head so that, when the rail contact is influenced by a master gage simulating a standard wheel, predetermined receiver output signals will be measured.